This invention relates to a programmable cardiac pacer and, in particular, a pacer with means and a method to vary the pacing rate in response to sensed heartbeat signals in a manner dependent upon one of a plurality of programmed modes and the timing of the sensed heartbeat compared to the prior pacer cycle or cycles.
Pacemaker design has evolved very rapidly over the last several years. There has been a great deal of work in enhancing the programmability of pacemakers, to enable them to be programmed to work with different selected operating parameters, and indeed to work in different modes. The evolution of pacemaker design has further led to the incorporation of a microprocessor into the pacemaker apparatus as a central means for providing flexible programming and control of the pacemaker operation. Incorporation of a microprocessor into cardiac pacers has been made possible by the development of a relatively low power microprocessor CPU as, for example, the CDP 1802. The CDP 1802 utilizes CMOS technology, and operates at a relatively low power with a voltage supply conveniently provided by an available lithium battery. For multiprogrammable pacemaker applications such a microprocessor is well suited for carrying out desired subroutines, interrupts and accessing program data, and provides good capability for software control of the pacer operation. Such a microprocessor is thus a highly suitable element in an overall design for a pacemaker which enables programming of multiple parameters of the pacer operation. Further, the speed and increased flexibility of microprocessor control enables cycle to cycle adjustments, permitting the pacer design to optimize one or more selected parameters, such as pacer rate and escape interval, in view of sensed cardiac conditions.
Control of pacing rate and escape interval are very important in a pacemaker system, for a variety of reasons. As used in this specification, the term pacing interval is the time interval between two consecutive delivered pacing pulses, while the term escape interval means the time that the pacemaker waits from a last sensed heartbeat until generation of a next pacing pulse. For a demand pacer, if a natural pulse occurs before the escape interval has timed out, the cycle is re-initiated, and timing of the escape interval starts over again. As used herein, the terms pacer cycle or cycle refer to the interval between a natural or paced heartbeat and the next natural heartbeat or delivered pacing signal.
In conventional demand pacers with hysteresis, the escape interval is greater than the pacing interval, thereby enabling the patient's natural pacemaker to continue to control the heart as long as the rate is maintained above a predetermined minimum.
Prior art pacemakers have recognized problems that can occur in the event of natural heartbeats at rates substantially above the normal programmed rate. In particular, such problems are acute when these changes are abrupt, e.g., when natural heartbeats suddenly occur at a rate far above the pacing rate. It is important that a pacemaker have means to stabilize irregular natural rhythms, such as occur with sick sinus syndrome. It is desirable to allow spontaneous activity to the extent possible, but it is likewise desirable to control such activity within safe limits. For example, the prior art shows the technique of keeping track of a spontaneous or natural heartbeat rate, but without intervening unless such rate goes above a predetermined maximum. While such tracking permits pacer capture at a rate corresponding to the last spontaneous rate when the spontaneous beats disappear, there is no limitation on the abruptness with which the spontaneous heartbeats are permitted to occur. On the contrary, it is frequently desirable to reduce the effect of premature ventricular contractions or random ectopics, and to continue pacing with relatively slower controlled changes in rate. Further, it is our observation that it is often preferrable not to adapt the pacing rate automatically to overcome or overdrive all heart irregularities, but rather to allow changes at any rate between prescribed minimum and maximum values so long as the rate of change is within allowable limits.
Another area of pacemaker rate control that is very important is that of adapting to sensed tachyarrhythmia, or dangerously high heartbeat rates. It is known that sometimes tachycardia, or even one or a few premature beats may induce cardiac fibrilation. It is further known that in many patients with sick sinus syndrome, exercise can produce dangerous high-rate heart irregularities. Studies have documented exercise-induced ventricular ectopy particularly in children and young adults with compete heart block. There have been a number of pacemaker designs which attempt to treat this situation, the common one being that of applying a burst of high-rate stimulus pulses to the patient's heart. Another system has been to immediately overtake the heart by delivering regular stimulus pulses at a rate higher than the observed spontaneous heartbeats.
Several prior art patents are of particular significance in providing the pertinent background of this invention. U.S. Pat. No. 3,693,626, Cole, discloses a pacer system wherein the natural rate is tracked as long as spontaneous heartbeats are occurring, i.e., the pacer keeps track are of the rate of the spontaneous beats. When the natural heartbeats cease, the pacer intervenes with a stimulus rate at or somewhat below the last detected rate, i.e., the pacing interval is only slightly longer than the interval between the last pair of spontaneous heartbeats. While this system avoids a discontinuity in heartbeat rate at the time that the pacer takes over, it does nothing to prevent discontinuities, or abrupt jumps in rate as long as spontaneous beats continue. In the U.S. Patent to Wall et al, U.S. Pat. No. 3,807,410, there is shown another analog tracking system wherein the pacer rate or escape interval is adapted to track, or follow the natural rate, and wherein the rate of returning to the fixed rate when natural beats disappear may be varied by the circuit design. However, this system likewise permits abrupt upward swings in rate without providing any control thereof.
The U.S. Pat. No. 3,857,399, to Zacouto, discloses a means of varying the pacemaker escape interval as a function of the measured time of the last cycle, but makes the escape interval dependent upon the absolute value of the spontaneous rate, and not the rate of change of the spontaneous rate. By contrast, this invention provides means for responding to the rate of change of the natural heartbeat action, including a controlled form of pacer overdrive of spontaneous pulses. The U.S. Pat. No. 4,163,451 to Lesnick et al discloses a form of immediate overdrive in view of any natural heartbeat, i.e., the pacing interval is made less than the interval of the last cycle. While such system may be useful in some particular cases, it is overly agressive for many applications. For example, it automatically causes pacer capture following a single premature ventricular contraction, or following a modest rise in the patient's pacer rate which is within a safe limit. It is desired to have a more controlled and less abrupt response which permits continuation of spontaneous heart activity under safe circumstances but yet insures pacemaker overdrive under proper circumstances.